Soft contact lenses are conventionally made of synthetic material, for example, hydrogels of hydroxyethyl polymethacryalate and/or of polyvinylpyrrolidone.
It has been proposed to make contact lenses from a natural collagen which is a protein constituting the intercellular substance of conjunctive tissue, or its derivatives, such as a gelatin, obtained by hydrolysis of the collagen.
These materials are sought because of their biocompatibility.
In order to be usable such materials must be cross-linked to create a polymeric network or system which is insoluble, notably in water. Therefore, in a first stage, they must be put into solution in order to give them a lens shape by pouring into molds having well-defined geometrical characteristics, then crosslinking is carried out which under normal wear conditions is irreversible.
Very high molecular weight collagens are difficult to make soluble and therefore they have not to date yielded products having sufficient mechanical properties.
Gelatin solutions, on the other hand, may include up to 30% polymer and have the well-known property of gelling by simply cooling. In French patent No. 77 38695 an acid collagen solution placed in a mold is crosslinked by irradiation.
In European patent application 0011523, filed July 14, 1982, a noncrystalline natural hydrogel of animal or vegetable origin is put into a solution in an acid medium. Usually it is gelatin. A crosslinking agent is added before introducing the mixture thus formed in the concave part of a mold.
A drying operation at a temperature less than 35.degree. C. is carried out until the amount of moisture has been reduced to 10%. The lens is then rehydrated in order to resume its configuration.
In this process the crosslinking of the lens is carried out at the same time as drying so that it is not possible to control reproducibility of such a process in an industrially satisfactory manner.
In U.S. Pat. No. 4,416,814 it is proposed to physically gel at low temperature a gelatin solution which does not contain any crosslinking agent though possibly agents contributing to physical gelation.
After opening the mold the lens must remain on one of the two parts of the mold, for example the convex part. It is then placed in a hermetic container with a crosslinking solution. All contact between the solution and the lens is carefully avoided so that only the vapor phase is operative in the crosslinking process which therefore is extremely time-consuming. Monitoring the hydrophilia of the lens during such a treatment is possible only by maintaining the partial pressure. There results an increased risk of modification of the dimensions of the lens since the lens necessarily adhere to one of two mold parts, the contact with the volatile matter is of course different on each of the faces.